1. Field of the Invention
The present invention generally relates to hydrogen generation fuel systems for a motor vehicle, and more specifically, to a fuel delivery and storage arrangement for a hydrogen fuel system.
2. Background Art
Generally, fossil fuels, namely hydrocarbons, provide the fuel required to operate the majority of the world's combustion engines. However, the shortage of hydrocarbons have led to the development of engine arrangements that use alternative fuel sources.
One alternative fuel source is hydrogen. Hydrogen is an attractive fuel source because it is the most abundant element in the universe and can typically fuel conventional engines with only minor modifications to the engines. Furthermore, hydrogen burns relatively pollution free, and a large weight percentage of the hydrogen can be converted to power engines as compared to, for example, gasoline. In addition to being used directly as a fuel in combustion engines, hydrogen can also be used in a variety of fuel cells through electrochemical oxidation.
Several methods of storing and/or generating hydrogen have been developed. One such method teaches the physical storing of hydrogen as a compressed gas or as a low temperature liquid in high pressure cylinders. However, liquefying the hydrogen requires a substantial amount of energy, obtaining and maintaining extremely low temperatures on a vehicle is very difficult, and fuel is lost over time due to evaporation and boil-off. In addition, the high pressure cylinders themselves pose problems due to their bulk and limited storage capacity.
In addition to the physical storage method described above, chemical methods of storing hydrogen have also been developed. One such chemical method includes reforming hydrogen containing fuels such as methanol or other hydrocarbons. However, this method requires a significant input of heat and does not solve the CO and CO2 emission problem due to the presence of carbon. Another chemical method available is the storing of hydrogen in reversible hydrides. However, this method is expensive, has very low storage efficiency by weight of hydrogen, and may require thermal energy to separate the hydrogen and the hydride. A third method is the production of hydrogen through the use of alkali metal hydride reactions. However, this method is associated with difficulties in controlling such reactions.
In order to overcome the problems of the above described methods for storing and producing hydrogen, it is currently known to utilize an aqueous solution to produce and store hydrogen. Using this approach, pure hydrogen is produced as required through the use of a catalytic reaction. The use of such an aqueous solution to store and generate hydrogen allows for the creation of a hydrogen powered fuel cell or combustion engine that operates in a safe and efficient manner and which can be easily refueled through the addition of more aqueous solution.
The aqueous solution employed in this approach is alkaline sodium borohydride (NaBH4, tetrahydroborate). When solutions of aqueous NaBH4 are reacted with metal or metal boride catalysts, these solutions hydrolyze to produce hydrogen gas and sodium borate which is water-soluble and environmentally safe. The overall reaction is:
            NaBH      4        ⁡          (      aq      )        +      2    ⁢          H      2        ⁢    O    ⁢                  ⁢          ς      catalyst        ⁢                  ⁢    4    ⁢          H      2        +            NaBO      2        ⁡          (      aq      )      
The above reaction is inorganic and yields no products which might harm the operation of a fuel cell, such as sulfur, carbon monoxide, or aromatics.
Stable NaBH4 solutions do not produce hydrogen unless contacted with a catalyst. While various metal salts may be employed as a catalyst in this hydrogen producing reaction, it has been found that the environmentally safe element ruthenium (Ru) is able to liberate hydrogen from borohydride solutions most rapidly. The Ru catalyst is supported on ion exchange resin beads and allows the above hydrogen producing reaction to proceed when brought into contact with the NaBH4 solution. This process allows hydrogen to be generated at close to ambient temperatures and is not plagued by the safety concerns of storing hydrogen through mechanical compression. In addition, because hydrogen is only produced as needed and because stabilized NaBH4 solutions do not produce hydrogen when not in the presence of a catalyst, no hydrogen will be generated in the event of a spill and no hydrogen will be released in the event of a puncture. NaBH4 solutions are nonflammable and can be easily stored in, for example, plastic containers. When NaBH4 is exposed to the atmosphere or water, there is no possibility of dangerous reactions occurring.
Borohydride solutions are also favored because they are able to satisfy the large energy demands of, for example, a motor vehicle. However, the use of borohydride solutions to supply hydrogen to power a motor vehicle also presents operating and design issues that must be addressed. For example, the conditions under which the hydrogen producing reaction takes place must be such that the temperature of the effluent is kept above a predetermined temperature so as to prevent precipitation. The predetermined temperature is approximately negative (−) 32° C. but varies slightly depending on molar concentration. Thus, a need exists for a vehicle fuel delivery arrangement capable of preventing the effluent of such a hydrogen producing reaction from precipitating under extreme ambient temperatures below negative 32° C.
A further issue relating to the use of a borohydride solution as a source of hydrogen fuel is that a waste liquid of NaBO2 remains after the catalytic operation. Such waste liquid is nonexplosive, and can be recovered and reprocessed into fresh borohydride solution. Thus, the fuel delivery system must account for storage of both the fresh borohydride fuel solution, as well as the resulting waste liquid. Such a requirement poses size and packaging problems when the system is used as a fuel source for a motor vehicle.